In manufacturing an LSI (large-scale integration), tungsten is widely used for a MOSFET gate electrode, a contact with a source/drain, a word line of a memory, or the like. In a multilayer interconnection process, a copper wiring is mainly used. However, copper is easily diffused and has insufficient heat resistance. Therefore, tungsten is used for a portion where heat resistance is required or a portion where electrical characteristics may deteriorate due to diffusion of copper.
A physical vapor deposition (PVD) method has been conventionally used for a tungsten film forming process. However, it is difficult in the PVD method to deal with a portion where a high step coverage is required. Therefore, a chemical vapor deposition (CVD) method that provides a good step coverage is used for film formation.
As a film forming method for forming a tungsten film (CVD-tungsten film) by using the CVD method, there is generally used a method in which reaction WF6+3H2→W+6HF occurs on a semiconductor wafer as a target object by using tungsten hexafluoride (WF6) as a source gas and H2 gas as a reduction gas (see, e.g., Japanese Patent Application Publication Nos. 2003-193233 and 2004-273764).
However, in the case of forming a CVD-tungsten film by using WF6 gas, a gate insulating film is reduced by fluorine contained in WF6 and electrical characteristics deteriorate at word lines of a memory or gate electrodes in a semiconductor device.
As a processing gas used for formation of a CVD-W film that does not contain fluorine, tungsten hexachloride (WCl6) is known (see, e.g., Japanese Patent Application Publication No. 2006-28572, and J. A. M. Ammerlaan et al., “Chemical vapor deposition of tungsten by H2 reduction of WCl6”, Applied Surface Science 53 (1991), pp. 24-29). Although both of chlorine and fluorine have reducibility, the reducibility of chlorine is weaker than that of fluorine. Further, it is desirable to decrease an adverse effect of chlorine on the electrical characteristics.
Along with the recent trend toward miniaturization of semiconductor devices, it is difficult to fill a pattern having a complicated shape even by using the CVD method that can provide a high step coverage. As a method capable of obtaining a higher step coverage, an atomic layer deposition (ALD) method, in which a source gas and a reduction gas are supplied sequentially with a purge process interposed therebetween, attracts attention.
However, in the case of forming a tungsten film by the ALD method using WCl6 gas as a source gas and H2 gas as a reduction gas, a deposited film thickness per one cycle is small. In other words, a film forming speed is low and, thus, a productivity is low.